An asynchronous transfer mode network ("ATM network") is made of a plurality of switches, each of which is coupled directly to at least one other switch. The points at which data enters and exits a switch are referred to as "ports."
ATM networks use virtual connections to send data across the network. A virtual connection is basically a path or a circuit between two switches in an ATM network that may be routed through any number of intermediate switches. Before sending a data packet to the next switch along the virtual connection, a switch attaches a header to the data packet that contains a virtual channel identifier and a virtual path identifier ("VCI/VPI"), which allows the next switch along the virtual connection to identify which virtual connection the packet is traversing. The VCI/VPI is unique to a particular virtual connection along a particular leg of the connection, but need not remain constant between different legs, i.e., the VCI/VPI may be changed at each switch.
A virtual connection may be set up for a particular data transmission having multiple data packets, such that each data packet in the transmission is sent along the virtual connection, which is then deactivated once the transmission is complete. Alternatively, a virtual connection may be a permanent virtual connection ("PVC"), which is used for multiple data transmissions.
Failures within an ATM network are typically resolved by a network management system ("NMS"). Each switch in the network is in contact with the NMS, and reports failures to the NMS. When a failure within the network is reported, the NMS reroutes virtual connections around the failure. The NMS may also have the capability to detect failures at the boundary or outside of the NMS's ATM network, but is typically not able to resolve such failures, particularly when networks made by different vendors are involved. Such a failure might occur at an interface, which is used to couple the ATM network to an entity external to the network, or at a port to which the interface is coupled. After such a failure is detected, the NMS may declare the interface and any PVC coupled to the interface in a failed state. Once a failed state is declared, manual intervention may be required to restore the PVC.
As ATM networks grow in popularity and are used in a wider variety of applications, interfaces between ATM networks and entities external to the ATM network are being created and used with increasing frequency. Accordingly, there is an increasing need for a way to resolve failures at these interfaces and the ports to which they are coupled.
Physical layer protection is one method of resolving failures at interfaces between networks, and the ports to which the interfaces are coupled. For example, an active interface between networks may be associated with a second interface reserved for use in case the active interface fails. This second interface may remain inactive until there is a failure, or the same data may be transmitted along both interfaces simultaneously such that a failure in one does not prevent transmission along the other. However, this type of physical layer protection approximately doubles the bandwidth required for a particular data transmission between networks, as it requires one interface to be dedicated to protection for each active interface. Alternatively, a single interface between networks may have a Y-cable or other means of splitting the signal at one or both ends, such that a failure after the split does not prevent transmission. However, this type of physical layer protection also uses excess bandwidth in that it requires four ports, two active and two dedicated to protection, for each active interface. In addition, this type of physical layer protection does not protect against a failure along the interface where the signal is not split.